1. Introduction
This invention relates to a sulfuric acid-hydrogen peroxide etchant and more particularly, to an etchant that provides a roughened, relatively porous etched surface.
2. Description of the Prior Art
The activation of relatively dilute mineral acids with an oxygen release compound is known in the art and described, for example, in Plating, "Surface Treatment of Metals with Peroxygen Compounds", Volume 42, page 561 (1955). In the Plating publication, it is taught that mineral acids such as sulfuric acid, nitric acid, and phosphoric acid may be activated with oxygen release compounds such as hydrogen peroxide or other peroxy, persulfate or perborate compounds. The oxygen release compound permits etching a metal with a decreased acid concentration. The most commonly used acid is sulfuric acid and the most commonly used oxygen release compound is hydrogen peroxide.
Sulfuric acid-hydrogen peroxide etchants are used to etch a variety of metals in many different processes. A common use of such an etchant is as a final etchant in the manufacture of printed circuit boards where an insulating backing is provided with a copper coating, 35 or 70 .mu.m thick, which is printed with an etch resistant printing ink, or photoresist, on the portions to be retained as copper lines after etching. After the resist has been applied, etching is affected by immersion or spraying the printed circuit base material with an etchant to dissolve the copper which is not protected by the resist. For such an application, the etchant is formulated to provide a rapid rate of dissolution so that all of the metal can be removed rapidly.
Etchants find uses other than as a final etchant to remove a full thickness of copper in the manufacture of printed circuit boards. For example, in the manufacture of double sided through-hole printed circuit boards, the copper cladding is first electrolessly and then electrolytically plated with copper, as are the walls of the through-holes before application of a resist pattern. Before electroless plating, the copper cladding must be properly prepared. Preparation typically includes the steps of cleaning and mildly etching the copper cladding. The purpose of cleaning and etching is to remove dirt and copper oxides so as to promote a strong bond between the copper cladding and the subsequently applied copper plate.
Ammonium persulfate etchants have been used in the prior art as a pre-etch to remove copper oxide and to etch copper preparatory to plating the copper cladding. The use of ammonium persulfate for this purpose is desirable as it provides an etched surface that, on a microscopic level, is pitted and porous whereby the surface area of the copper cladding is increased. The increased surface area enhances the bond between a subsequently applied copper plate and the copper cladding. There are also disadvantages to the use of an ammonium persulfate etchant, the most notable one stemming from the instability of the persulfate and the need for waste treatment of ammonium salts. In use, ammonium persulfate decomposes within several days of its use and must be replaced with fresh solution. This, coupled with the requirement for waste treatment, adds to the cost of manufacture of the printed circuit boards.
Sulfuric acid-hydrogen peroxide etchants can be stabilized so that they are capable of use for extended periods of time and such etchants have been previously used for preparation of copper cladding for subsequent electroless copper deposition. However, good adhesion between a subsequently applied electroless copper plate and the copper cladding is unreliable using the prior art etchants, particularly when using copper plating solutions capable of rapidly depositing copper.